A245044 Semiprimes with digits in descending order that differ exactly by 2.

4, 6, 9, 86, 753, 7531, 9753, 97531

Offset

1,1

Links

Table of n, a(n) for n=1..8.

Example

7531 is in the sequence because it is semiprime 7531 = 17*443 and 7-5 = 5-3 = 3-1 = 2.
97531 is in the sequence because it is semiprime 97531 = 7*13933 and 9-7 = 7-5 = 5-3 = 3-1 = 2.

Mathematica

Sort[Flatten[Table[Select[FromDigits/@Partition[Range[9, 0, -2], n, 1], PrimeOmega[#]==2 &], {n, 1000}]]]

Prog

(PARI) for(d=1, 5, for(s=1, 11-2*d, t=sum(i=1, d, (s-2+2*i)*10^i)/10; if(bigomega(t)==2, print1(t", ")))) \\ Charles R Greathouse IV, Jul 16 2014

Crossrefs

Cf. A215477, A215478.

Sequence in context: A086698 A317248 A115666 * A104389 A115655 A363248

Adjacent sequences: A245041 A245042 A245043 * A245045 A245046 A245047

Keyword

nonn,base,easy,fini,full

Author

Vincenzo Librandi, Jul 10 2014

Extensions

a(1), a(2), a(4) from Charles R Greathouse IV, Jul 16 2014

Status

approved